The neuropathological features of Alzheimer's disease are characterized by the presence of insoluble amyloid deposits in the brain and cerebrovasculature, the intra-neuronal accumulation of abnormally phosphorylated and aggregated forms of tau, a microtubule binding protein and neuronal loss. Although the exact mechanisms producing these pathological changes remain unknown, the amyloid cascade hypothesis states that the peptides (Ass) that make up amyloid deposits are the cause of the disease process. Despite numerous supportive studies, discrepancies between animal models of AD and humans with AD remain an obstacle for full acceptance of Ass as the primary causal agent for AD. By altering the nitric oxide in mouse brain, we have generated a novel mouse model that provides unique insights into mouse-human differences. Our bigenic mouse models of AD increase the expression of human Ass on a murine nitric oxide synthase 2 (NOS2) knockout background. The resulting phenotype is highly reminiscent of the pathology observed in humans with AD including high levels of Ass peptides, tau hyperphosphorylation, tau redistribution and tau aggregation, neuronal loss and behavioral deficits. A primary advantage of the APPSw/NOS2-/- mouse is the formation of tau pathology from normal, not mutated tau AND the presence of significant neuronal loss. Thus, our model provides a unique opportunity to fully test the amyloid cascade hypothesis in vivo under conditions of chronic disease. The first aim will confirm a pathological cascade in the APPSw/NOS2-/- mouse brain by measuring Ass, tau pathology, neuronal loss and memory and learning in the APPSw/NOS2-/- mice brains at specific ages. To establish a direct, causal role for Ass peptides in the cascade, we propose a) to reduce Ass levels in the brains of APPSw/NOS2-/- mice by passive immunization and b) to increase Ass levels in the brains of NOS2-/- mice using intrahippocampal injection of Ass peptide mixtures. The second aim will examine the role of NOS2. We propose a) to reduce brain iNOS protein using lentivirus delivery of small interfering RNA (shNOS2 lentivirus) b) to test the ability of NOS2 and NO replacement to alter the pathological cascade mediated by Ass. The third aim will examine a likely mechanism of NO's action, the regulation of caspase activity.